sensdemo.icn: Program to demonstrate sensor routines

   sensdemo illustrates several of the input sensors provided in the
program library.  It is written to use mutable colors but will struggle
along slowly if they're not available.

   There are four pushbuttons.  Buttons "One", "Two", and "Three" just
write a line on standard output.  The "QUIT" button does what you'd
expect.

   The three vertically oriented sliders control (from left to right)
alter the red, green, and blue components of the color in the large
square.  The individual components appear in the small squares, and
the hexadecimal form of the color spec is displayed below the square.

   The small horizontal slider below the square adjusts all three
color components simultaneously.  Notice how moving it also moves
the three vertical sliders.

   The largs square sounds a bell if Return is pressed while it
contains the cursor.  The standard "quitsensor" causes the program
to exit when q or Q is pressed anywhere in the window.